7.2 Backward Compatibility

Code compiled with older versions of cl-defstruct that
doesn't use records may run into problems when used in a new Emacs.
To alleviate this, Emacs detects when an old cl-defstruct is
used, and enables a mode in which type-of handles old struct
objects as if they were records.

— Function: cl-old-struct-compat-mode arg

If arg is positive, enable backward compatibility with old-style
structs.